Original virtqueue design allows queued and out-of-order processing, but
helpers added in r253440 suppose only direct blocking in-order one.
It could be fine for network, etc., but it is a huge limitation for storage
devices.
Call through to /dev/random synchronously to fill
virtio buffers with RNG data.
Tested with FreeBSD-CURRENT and Ubuntu guests.
Submitted by: Leon Dang
Discussed with: markm
MFC after: 3 weeks
Sponsored by: Nahanni Systems